The present invention relates to fiber optic communication systems, and more particularly to apparatus for optically transmitting a plurality of RF subcarriers in adjacent octaves.
Cable television systems currently distribute television program signals via coaxial cable, typically arranged in tree and branch networks. The use of television signals comprising amplitude modulated vestigial-sideband video subcarriers is preferred in the distribution of cable television signals due to the compatibility of that format with National Television Standards Committee ("NTSC") television standards and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission, however, is that it requires a much higher carrier-to-noise ratio ("CNR") than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
One problem in implementing an optical fiber distribution system, particularly for AM-VSB signals, is that directly modulated semiconductor lasers of the type typically used in fiber optic systems produce high levels of distortion products (particularly second order) that are not compatible with multi-channel AM-VSB video signals. This is due to the short lifetime of the carrier excited state within the semiconductor laser. A recombination time of such a laser operating near 1.3 .mu.m or 1.5 .mu.m is about 1.2 nanoseconds, which is short compared to the period of a typical AM-VSB subcarrier operating in the cable television band of about 55.25 MHz to 1 GHz.
The dependence of second order distortion on carrier lifetime in a semiconductor laser used as an optical amplifier is discussed in A. A. M. Saleh, et al., "Nonlinear Distortion Due to Optical Amplifiers in Subcarrier-Multiplexed Lightwave Communications Systems", Electronics Letters, Vol. 25, No. 1, pp. 79-80, 1989. As noted in that article, second order nonlinear distortion is a significant problem in proposed lightwave cable television home distribution systems.
The difficulties presented in transmitting multi-channel AM-VSB television signals over fiber optic distribution systems have led others to propose the use of frequency modulation ("FM") instead of the more desirable AM-VSB format. See, e.g., R. Olshansky, et al., "Microwave-Multiplexed Wideband Lightwave Systems Using Optical Amplifiers for Subscriber Distribution", Electronics Letters, Vol. 24, No. 15, pp. 922-923, 1988; R. Olshansky, et al., "Subcarrier Multiplexed Passive Optical Network for Low-Cost Video Distribution", presented at OFC 1989; and W. I. Way, et al., "Carrier-to-Noise Ratio Performance of a Ninety-Channel FM Video Optical System Employing Subcarrier Multiplexing and Two Cascaded Traveling-Wave Laser Amplifiers", presented at OFC 1989. Another proposal has been to convert AM-VSB signals to a digital format for transmission. Digital transmission of AM-VSB television signals over an optical communication link is described in U.S. Pat. No. 4,183,054 to Patisaul, et al., entitled "Digital, Frequency-Translated, Plural-Channel, Vestigial Sideband Television Communication System".
It would be advantageous to provide an optical fiber based cable television distribution system that utilizes the AM subcarrier modulation format and frequency plan utilized by current coaxial cable distribution systems. Such a fiber based system would provide compatibility with existing coaxial systems, and enable an orderly progression from current coaxial systems to hybrid coaxial/fiber systems, and ultimately to all fiber distribution systems.
The present invention provides optical fiber based distribution apparatus for AM subcarriers that enjoys the aforementioned advantages and overcomes the problem of second order distortion products generated by directly modulated semiconductor lasers.